Hydraulic cylinders utilize pressurized hydraulic fluid to produce linear motion and force. A single action hydraulic cylinder is pressurized for motion in only one direction, either pulling in or pushing out. When a hydraulic cylinder is configured for pushing out, an external retraction spring is often used for rod return when the hydraulic pressure is removed. Single action hydraulic cylinders in combination with retraction springs are commonly used for positioning the plow blades of a typical V-type snowplow.
When employed for positioning snowplow blades, it is desirable that a single action hydraulic cylinder is capable of performing four functions: (1) extension; (2) retraction; (3) locking; and (4) relief of excess pressure when locked. Extension and retraction allow the blades of a typical V-type snowplow to be positioned back in a “V” position, forward in a “scoop” position, or in-line with each other in a “straight position”.
Extension is effected by the continued application of hydraulic fluid pressure to the interior of the hydraulic cylinder, usually through a fluid port at one end of the cylinder. The mounting fluid pressure translates into linear motion as the piston, and the rod to which it is coupled, is pushed by the fluid pressure along the axis of the cylinder, such that the rod is forced out from the cylinder. Accordingly, a plow blade, to which the rod is connected, is pushed forward. Retraction is effected by the combination of the opening of the fluid port and the force of the retraction spring pulling the blade back. Once the blade is set in a desired position, the fluid port is closed, thereby holding constant the volume of hydraulic fluid in the cylinder. The external force of a retraction spring causes the hydraulic cylinder to be “locked” in position. As the plow blade pushes snow forward, the fixed volume of fluid in the pressurized cylinder resists the force of the snow mass pushing against it.
As the piston moves in the cylinder during extension and retraction, the volume of fluid behind the piston (V1) and the annular volume of fluid surrounding the rod on the opposite axial side of the piston (V2) change. During extension, V1 increases while V2 decreases. During retraction, V1 decreases while V2 increases. To accommodate the fluid volume changes, hydraulic cylinders are commonly configured in a hydraulic circuit, and utilize one or more direction control valves to route the fluid accordingly. Without such means for routing the fluid in or out of each area of the hydraulic cylinder, the fluid volumes would remain fixed, and the cylinder would remain locked in position, unable to extend or retract.
Locking a single action hydraulic cylinder is desirable in some situations. When a plow blade pushes a mass, such as snow, forward, it is crucial that the hydraulic cylinder resist the force of the mass and not retract. As stated above, the closure of the hydraulic port provides a fixed volume of hydraulic fluid, through which the piston cannot travel. However, when the dragging a snow mass backwards with the back of a plow blade, also known as “back-blading”, only the force of the retraction spring is present to prevent the plow blades from being pulled forward by the mass, which often is insufficient, and the snow load is lost. It is, therefore, desirable to include a means for locking the cylinder while back-blading.
When the cylinder is locked, an excessive pressure spike on the cylinder may result in a hydraulic line/seal rupture and damage to other pressure vessels and equipment. If the cylinder is capable of locking while back-blading, an excessive pressure spike may occur, for example, if a blade hits an obstruction such as a curb or an irregularity in the ground surface. Commonly, relief valves are used to release excessive pressure. These valves control or limit the pressure in a system by allowing the pressure source fluid to flow from an auxiliary passage, away from the main flow path. A relief valve is designed to open at a predetermined pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits. When the pressure setting is exceeded, the relief valve becomes the “path of least resistance” as the valve is forced open and a portion of the fluid is diverted through the auxiliary route. In the event of excessive pressure while back-blading, a relief valve would allow the cylinder to become unlocked and extend, thereby allowing the obstructed blade to be pushed forward in response to the force of the obstruction.
Direction control valves are designed to open under normal operating fluid pressures for extension and retraction. Relief valves are designed to open in response to excessive pressure, such as the excessive pressure spike described above. As such, the fluid pressure required to open a direction control valve in a hydraulic cylinder is significantly lower than the fluid pressure required to open a relief valve in the same cylinder. U.S. Pat. No. 6,134,814 (Christy) teaches, in one embodiment, a hydraulic locking cylinder capable of extension, retraction, locking, and relief of excessive pressure. The Christy cylinder includes a valve assembly coupled to the piston comprising a check valve, which is a type of direction control valve that allows flow in only one direction, coupled to a pilot assisted relief valve. By including a direction control valve and a relief valve in the valve assembly, the valve assembly is capable of performing the direction control and relief functions, as well as the cylinder locking function desired for back-blading.
The Christy valve assembly requires complex machining and the assembly of separate valves and other cartridge components. As such, the Christy valve assembly has a greater likelihood of failure than a valve comprised of fewer and simpler components, due to the greater margin of error that exists in the manufacture and assembly of a greater number of components. Valves utilized in hydraulic cylinders are often exposed to repeated stress. A valve assembly comprised of fewer and simpler components would likely have a longer life and cost less to manufacture:
Therefore, there is a longfelt need for a hydraulic cylinder valve that is capable of performing the functions of a direction control valve and a relief valve, has fewer components, fewer failure modes, a durable structure, and is as easy to install, yet less expensive to manufacture, than the Christy invention.